1. Field of the Invention
The present invention is related to spread spectrum wireless receivers and more particularly, to signal extraction circuits for code division multiple access (CDMA) receivers such as cellular telephones.
2. Background Description
Cellular telephones (cell phones) communicate using what is known as code division multiple access (CDMA) protocol. Call information is encoded with a unique pseudo-random code, also called pseudo-noise (PN). The encoded information modulates a carrier signal that is amplified and transmitted between network stations, e.g., between a base station or Radio Carrier Station (RCS) and one or more cell phones or Fixed Subscriber Units (FSU). Each unique pseudo random code has high auto correlation and low cross-correlation with other PNs. Thus, a receiver can use the same PN with which a signal is encoded to extract the signal from a band of signals and reject the rest, each of which is based on a different PN code.
However, very often the source of the signal being extracted from the band is much weaker than other signals in the same band or in adjacent bands, such that the others interfere (“interferers”) with the signal. In fact, interferer strength may be as much as 60 dB or more above the signal. Accordingly, a good CDMA receiver must reject the interferers before it can extract the signal. A typical state of the art super-heterodyne radio frequency (RF) receiver includes an intermediate frequency (IF) stage with a surface acoustic wave (SAW) filter that rejects adjacent interferers, often by more than 35 dB. Usually, baseband signal processing rejects any remaining interferer signals. The IF filter achieves acceptable phase linearity and satisfactory performance at the expense of higher cost and complexity.
A simpler, lower cost approach, a direct conversion receiver (DCR), however, does not contain an IF stage. Instead, a DCR converts the RF input signal directly to a baseband signal. DCRs use additional analog baseband rejection to compensate for the omission of the IF filter. Unfortunately, this omission results in DCR analog baseband rejection that fails short of filtering comparable to a SAW filter. Also, DCR analog baseband filters have much poorer phase linearity, which somewhat undermines the advantages of using an inexpensive or less expensive DCR. Consequently, adding an analog phase equalizer is one approach to linearizing the phase response of the DCR analog baseband.
Unfortunately, besides consuming additional power, analog phase equalization is sensitive to component variations from temperature changes, aging, etc. So, while phase equalization has some advantages, it still may not solve the DCR phase linearity problem because the added phase equalizer would still exhibit phase response variations from component and ambient conditions that are inherent in any analog circuit. Also, because analog phase equalizers normally have fixed transfer function coefficients, an analog phase equalizer cannot be changed or adapted to compensate for these variations.
Thus, there is a need for a DCR with phase linearity and performance comparable to that achieved from an IF stage in a traditional super-heterodyne receiver.